A heterogeneous network (HetNet) is a network that includes macro cells and low-power nodes such as pico cells, femto cells, and relays. The low-power nodes or small cells are frequently overlaid on top of macro cells, possibly sharing the same frequency. These small cells may offload macro cells, improve indoor and cell edge performance, or provide other advantages. 3GPP studies for LTE-Advanced (Release 10) include HetNet deployments as a major performance enhancement enabler. In HetNet deployments, inter cell interference coordination (ICIC) plays an essential role, and time domain based resource sharing or coordination has been adopted as enhanced ICIC (eICIC), which includes Almost Blank Subframe (ABS) based solutions. LTE-Advanced (LTE-A) identifies two main deployment scenarios where eICIC is utilized. In a first or CSG (Femto cell) scenario, dominant interference condition may occur when non-member users are in close proximity of a CSG cell. In some instances, downlink transmission from the nonmember CSG cell may significantly interfere with the Physical Downlink Control Channel (PDCCH). Interference to the PDCCH of the macro cell may have a detrimental impact on both uplink and downlink data transfer between the UE and the macro cell. In addition, the downlink transmission from the nonmember CSG cell may also interfere with other downlink control channels and reference signal, which may originate from both the macro cell and neighbor cells and may be used for cell measurements and radio link monitoring. Depending on network deployment and strategy, the system may not be able to divert the users suffering from inter-cell interference to another E-UTRA carrier or other Radio Access Technology (RAT). Time domain ICIC may be used to allow such nonmember UEs to remain served by the macro cell on the same frequency layer. The interference may be eliminated, minimized, or otherwise reduced by the CSG cell utilizing Almost Blank Subframes (ABSs) to protect the corresponding macro cell's subframes from the interference. A non-member UE may be signaled to utilize the protected resources for radio resource measurements (RRM), radio link monitoring (RLM) and Channel State Information (CSI) measurements for the serving macro cell, allowing the UE to continue to be served by the macro cell under strong interference from the CSG cell.
In a second or Pico scenario, time domain ICIC may be utilized for pico cell users who are served on the edge of the serving pico cell such as for traffic off-loading from a macro cell to a pico cell. Typically, downlink transmissions from the macro cell may severely interfere with the PDCCH. In addition, the downlink transmission from the macro cell may also interfere with other downlink control channels and reference signals from both the pico cell and neighbor pico cells. The other downlink control channels and reference signals may be used for cell measurements and radio link monitoring. Time domain ICIC may be utilized to allow such UEs to remain served by the pico cell on the same frequency layer. This interference may be reduced by the macro cell(s) utilizing ABSs to protect the corresponding pico cell's subframes from the interference. A UE served by a pico cell may use the protected resources for radio resource measurements (RRM), radio link monitoring (RLM) and Channel state information (CSI) measurements for the serving pico cell.
For the time domain ICIC, subframe utilization across different cells are coordinated in time through backhaul signaling or operations and management (OAM) configuration of so called ABS patterns. In general, the ABSs in an aggressor cell are used to protect resources in subframes in the victim cell receiving strong inter-cell interference from the aggressor cell. ABSs are subframes with reduced transmit power (including no transmission) on some physical channels and/or reduced activity. The eNB ensures backwards compatibility towards UEs by transmitting necessary control channels and physical signals as well as system information. Patterns based on ABSs are signaled to the UE to restrict the UE measurement to specific subframes, called time domain measurement resource restrictions. Different patterns may be implemented depending on the type of measured cell (serving or neighbor cell) and measurement type (e.g. RRM, RLM). In some cases, the macro eNB (the aggressor) configures and transfers the ABS patterns to the pico eNB (victim). The macro eNB does not schedule data transmissions in ABS subframes to protect the UEs served by the pico eNB in the edge of the pico cell. The pico eNB may schedule transmission to and from the UEs in the cell center regardless of the ABS patterns. Meanwhile, the pico eNB may schedule transmission to and from the UEs in the edge of the cell only in ABSs. The pico cell may configure the UE which is in the edge of the cell with three different measurement resource restrictions independently based on the received ABS pattern. The first restriction may be for RRM measurement and radio link monitoring (RLM) for the PCell (in this case the serving pico cell on the primary frequency). If configured, the UE measures and performs RLM of the PCell only in the configured subframes. The second restriction is for RRM measurement of neighbor cells on the primary frequency. If configured, the UE measures neighbor cells in the configured subframes only. The restriction may also contain target neighbor cells to which the restriction will be applied. The third restriction is for channel state estimation of the PCell. If configured, the UE estimate CSI and CQI/PRM/RI in the configured subframes only.
According to the current Radio Resource Control (RRC) protocol specification, MeasSubframePatternConfigNeigh is an optional (need ON) information element (IE) within the EUTRA measurement object, measObjectEUTRA. The phrase “need ON” means that, in case the information element is absent, the UE takes no action and, where applicable, continues to use the existing value (and/or the associated functionality).
In some scenarios, intra frequency handover may switch the UE to an area managed by the Release 8 or 9 eNB or LTE system or LTE-A system without eICIC/HetNet features as opposed to an LTE-A system with support of eICIC/HetNet features. The target eNB prepares the handover command, but the target eNB does not support the time domain measurement resource restriction. The target Release 8 or 9 eNB is not capable of indicating release of the restriction in the prepared handover command or releasing the restriction by a reconfiguration message after the handover without utilizing the full configuration option, by which all radio configurations including measurement configurations are released. With the full configuration options, the size of the reconfiguration messages is larger than without the option. Therefore, the use of this option should be limited to maintain efficient operations. Also, the target LTE-A eNB without eICIC/HetNet features may not instruct the UE to release the measurement restrictions. Then the UE would continue to apply the restriction because the measSubframePatternConfigNeigh is not included in the handover command. In the intra frequency handover example, the measurement resource restriction for neighbor cells may be applied incoherently, i.e., the UE applies the restriction but the eNB does not. The incoherent application may result in unintended handover or radio link failure due to the difference in performance requirements (RSRP and RSRQ accuracy and detection time of neighbor cells) with or without the measurement resource restrictions for neighbor cells. With the measurement resource restriction, better signal to interference and noise ratio (SINR) is required to have the same RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality) accuracy than without the restriction, so the measurement results reported by the UE applying the restriction may be less accurate than the eNB expects, which may result in an unintended handover. In addition, it may take longer time to detect a neighbor intra frequency cell with the measurement restriction, which may results in radio link failure. The source eNB may perform measurement reconfiguration while preparing the handover with the target eNB. However, due to this additional processing, the handover execution may be delayed, which results in higher handover failure rate.
With regard to inter-frequency handover cases, after inter-frequency handover (f1 to f2), time domain measurement resource restriction configured for the source primary frequency (f1) is not applied to f1 (since f1 is not the primary frequency anymore after the handover) but maintained (not released) unless explicitly done so by RRC Connection Reconfiguration message. Depending on the release of target eNB or support of eICIC/HetNet features, the time domain measurement resource restriction for neighbor cells may be applied incoherently, i.e., the UE applies the restriction but the eNB does not. The incoherent application may result in unintended handover or radio link failure as explained above.
In some scenarios, after the handover (f1 to f2) mentioned above, the subsequent inter-frequency handover (f2 to f1) brings the UE to the area managed by the Release 8 or 9 eNB or the LTE-A eNB without eICIC/HetNet features. The target eNB prepares the handover command. In this case, the target eNB does not support the time domain measurement resource restriction. The target Release 8 or 9 eNB is not capable of indicating release of the restriction in the prepared handover command or releasing the restriction by a reconfiguration message after the handover without utilizing the full configuration option, by which all radio configurations including measurement configurations are released. With the full configuration options, the size of the reconfiguration messages is larger than without the option. Therefore, the use of option should be limited to maintain efficient operations. Also, the target LTE-A eNB without eICIC/HetNet features may not instruct the UE to release the measurement restrictions. Then, the UE restarts to apply the restriction for f1 upon the handover. The issue above is also applicable to the case when the UE reestablishes the RRC connection. The time domain measurement resource restriction for neighbor cells may be applied incoherently, which may result in another radio link failure or unintended handover.
In some scenarios, after experiencing radio link failure, the UE may find a suitable cell and reestablishes the RRC connection in the cell controlled by release 8/9 eNB or LTE-A eNB without eICIC/HetNet features. In this case, the subsequent eNB does not support the time domain measurement resource restriction. The subsequent Release 8 or 9 eNB is not capable of indicating release of the restriction in the by a reconfiguration message after the reestablishment without utilizing the full configuration option, by which all radio configurations including measurement configurations are released. With the full configuration options, the size of the reconfiguration messages is larger than without the option. Therefore, the use of option should be limited to maintain efficient operations. Also, the subsequent LTE-A eNB without eICIC/HetNet features may not instruct the UE to release the measurement restrictions. Then, the UE will start to apply the restriction upon the reestablishment. This issue applies irrespective of the frequency of the cell on which the UE reestablishes the RRC connection.
In some scenarios, the UE reestablishes the RRC connection in the cell where the restriction is no longer applied. The eNB releases the restriction by sending a reconfiguration message after the reestablishment procedure but the message transmission delays due to heavy load on the eNB or the message does not reach to the UE due to bad radio condition. Meanwhile, the UE restarts to apply the restriction. This issue applies irrespective of the frequency of the cell on which the UE reestablishes the RRC connection.
Like reference symbols in the various drawings indicate like elements.